A fast tool servo is a well-known device that can be added to a new or existing machine tool to provide an additional axis of motion between the cutting tool and a workpiece. A fast tool servo most notably distinguishes itself by its ability to move the tool at a much higher bandwidth, that is at a high speed of controlled, repetitive motion, on its axis relative to the other machine tool axes, with accuracy equal to or better than that of the other tool axes. Fast tool servos fall into two broad categories: rotary and linear. A rotary fast tool servo produces relative motion between the cutting tool and a workpiece by rotation of a swing arm that carries the tool at a fixed radius from the axis of rotation. A linear fast tool servo produces relative motion between the cutting tool and a workpiece by producing a linear translation of the tool.
A rotary fast tool servo is preferred in certain precision machining applications that are intolerant to the reaction force developed by a linear fast tool servo. For instance, in an application where it is desired to produce a textured surface on a spherical-shaped workpiece a fast tool servo is mounted on a rotary table that allows the tool to engage the workpiece, which is mounted to a spindle, at all points from its “pole” to its “equator”. A rotary-type mechanism oriented with its rotation axis parallel to the rotation axis of the rotary table generates a reaction torque on the rotary table, which can be allowed to rotate freely as a reaction mass or be locked and allowed to transmit the torque to the machine structure. In the first case motion of the rotary table is insignificant in most cases. In the later case the machine structure experiences a disturbance torque whose value does not depend on the angle of the rotary table. In contrast, a linear fast tool servo generates a reaction force on the rotary table. This is generally not a problem when the rotary table is positioned so that the reaction force is parallel to the direction of travel of the slide carrying the rotary table. However, when the rotary table is positioned so that a component of the reaction force is perpendicular to the direction of travel of that slide, that force component is transmitted by the slide to the machine structure as a disturbance. To the extent that the tool/workpiece interaction is affected by disturbances to the machine structure, the linear fast tool servo will produce errors in the desired surface texture as a function of “latitude” on a spherical workpiece if the reaction force is not dealt with.
Current fast tool servo technology does not support sufficient bandwidth to meet certain manufacturing goals and is also not sufficiently fast to machine certain types of materials, for example, some plastics, properly. It is desirable to have a method and apparatus for a rotary fast tool servo having a higher bandwidth than currently available systems.